The present invention relates to a cured adhesive composition and method for providing a water-tight seal to joints of roofing membranes, and more particularly to an EPDM-based adhesive and method for joining sheets of EPDM rubber roofing material together.
Roofing materials for covering large roof areas are customarily prepared in wide sheets for installation. Elastomeric ethylene-propylene-nonconjugated diene terpolymer (EPDM) and isobutylene-conjugated diene copolymer (butyl rubber) compositions are well known in the art as roofing materials due to their barrier properties against moisture. These sheets must be overlapped and spliced together to form a continuous, water-tight sheet which covers a roof.
EPDM roof membranes are manufactured to industry standards which permit only a 2% shrinkage in any dimension. Much of the shrinkage is caused by tension when the sheet is calendared and wound on a core and cured. As the EPDM sheet is unwound it recovers, causing the shrinkage. The recovery or shrinkage is dependent on the amount of stress built in during processing (calendaring and winding). If the sheet recovers 2% in a 100 foot roll it will shrink two feet. Seams in EPDM membranes are typically 5 inches for adhesive glues and as narrow as 2.5 inches for tapes. After seaming is accomplished on the roof site, ensuing shrinkage could pull the seams apart if the tape lacks sufficient static load strength.
The first generation of tapes introduced to the industry were uncured compositions that contained curatives. Vulcanization was achieved only after extended exposure to high roof top temperatures. It has been witnessed in roof curing (in situ curing) tapes that sliding of the seam in the shear direction can occur before there is enough time for the adhesive to cure and gain sufficient strength to hold the rubber sheets, and thus the seam, intact.
Besides the normal shrinkage of the EPDM membrane, large variations in temperature can occur shortly after the roof is installed. If the EPDM membrane is seamed during the day while the EPDM membrane is hot, the contraction of the EPDM membrane when the temperature falls at night could result in early seam failure before the adhesive cures. In the spring or fall, the EPDM membrane temperature could change as much as 38.degree. C. from night to day. During the manufacturing of EPDM sheeting stresses are built into the sheet as described above. When the sheet is subsequently rolled out on a roof surface, recovery occurs resulting in reduction in size. The amount of "shrinkage" that results determines the stress forces applied to the bonded seams.
Close, U.S. Pat. No. 4,472,119, is an example of an uncured roofing adhesive composition. The composition is taught to be applied as a liquid dissolved in a solvent. Example I shows a composition which is cured in situ for seven days after application to a roofing membrane. However, the prior art has recognized the shortcomings of such in situ cure adhesives. Chiu, U.S. Pat. No. 4,588,637, in the paragraph bridging columns 1 and 2, describes the problems with uncured adhesive tapes including low initial strengths. Metcalf, U.S. Pat. No. 4,601,935, also describes the shortcomings of in situ curable adhesives, including low initial strengths.
Another method used previously in the art for sealing together the overlapping sheets of roofing material has been to position an unvulcanized rubber tape between the overlapped portions of the roofing membranes and then spot vulcanizing the tape by the application of heat and pressure. However, this method required the presence of a vulcanizing press on the job site. Further, good adhesion required long vulcanizing times which slowed down the installation of the roofing materials.
Yet another method of sealing the overlapping sheets together has been the use of adhesives such as solvent-based neoprene-based adhesives. Problems have been encountered not only with the strength of the adhesive bonds formed at the splices, but also with the long term durability of those bonds. For example, environmental conditions may act to impair the quality of the bond achieved. If conditions are windy, dust and other contaminants may become lodged in the adhesive and impair its ability to adhere the sheets of roofing material together. If there are high temperatures, the adhesive may dry out too quickly. High humidity may cause moisture condensation which interferes with good adhesion. Variations in environmental conditions may require that the workers installing the roof modify their procedures, complicating the installation.
The use of brushes that leave streaks or coatings of uneven thickness are known to reduce bond strength. Neoprene adhesives contain solvents, usually aromatic, such as toluene, xylene, and others. Solvents are environmentally undesirable and subject to increasing regulation. They have toxicity and pose a health hazard. Finally, due to their flammability, a fire hazard exists, and there have been many instances in the industry where fires and injuries have occurred as a result of the use of solvent-based products.
Attempts have been made in the art to develop better adhesives which may be applied more readily and which provide a long term capability to withstand moisture penetration. For example, Streets, U.S. Pat. No. 4,640,730, teaches the use of a styrene-butadiene block copolymer mixed with a hydrocarbon resin as an adhesive for EPDM and butyl rubber-based roofing materials. Chiu, U.S. Pat. Nos. 4,588,637 and 4,855,172, teach a roofing adhesive of a cured butyl rubber-based composition made by compounding a butyl rubber copolymer, a curing agent for the butyl rubber, carbon black, and a compatible tackifier.
Kakehi, U.S. Pat. No. 4,404,056, teaches a cold vulcanizable adhesive tape having a Mooney viscosity of from 5 to 25 which includes a rubbery polymer, a vulcanizing agent, a vulcanizing accelerator, an adhesive agent, and a softening agent. The tape is positioned between overlapping sheets of roofing material, and the sheets are pressed together with a roller or the like.
However, the need still remains in the art for effective adhesive compositions and methods of application for use in adhering together sheets of EPDM-based roofing materials which provides not only ease of application and good strength, but also a long term water-tight seal.